Traveling wave tubes of circular structure



Nov. 5, 1957 G. MOURIER TRAVELING WAVE TUBES OF CIRCULAR STRUCTURE Filed Feb. 24, 1954 R m m I ATTOP/VEK United States Patent ()fiiice 2,812,473 PatentedNov. 5, 1957 WAVE TUBES OF CIRCULAR STRUCTURE Georges Mourier, Paris, France, assignor to Compagnie Generale de Telegraphic Sans Fil, a corporation of France TRAVELING Tubes of circular structure, with crossed electric and magnetic fields are known for instance from U. S. Patent No. 2,511,407 of December 27, 1947.

Such tubes otter definite advantages as compared with rectilinear tubes of the same type. In particular, power and gain being equal, they are more compact.

However, the very fact that they are of circular structure results in a number of disadvantages.

One of the essential conditions to be fulfilled, if these tubes are to operate satisfactorily is the following: the mean velocity of the electron beam propagating in the interaction space must be substantially equal to thephase velocity of the U. H. F. field generated by the wave traveling along the delay line which constitutes the anode of the tube. Now, as is well known, the same mean velocity of the electron beam is equal in a suitable system of units,

to E/B, where E is the intensity of the D. C. electric field andB the magnetic field prevailing in the interaction space.

Contrary to what'occurs in a rectilinear structure tube, the electric field in the interaction space of'a circular tube is notauniform one. As a matter of fact, theinteraction space may be considered as a cylindrical condenser; at any given 'point of the interaction space, the electric field is'inversely proportional to the distance between that point and the axis common to the two electrodes which constitute the armatures of the above condenser.

Therefore, the electrons are subjected to electric fields which are functions of their instantaneous distance to the axis of the tube.

qAs aresult, the Velocity of the electrons varies-as a function of their instantaneous distance to the axis of the tube.

Moreover, the phase velocity of the U. H. F. field at any point of the interaction space is also dependent on the distance between that point, and, therefore, the axis of the tube and the operation isdisturbed.

It is an object of the present invention to provide an improved traveling wave tube of circular structure, in which the aforementioned disadvantage is substantially eliminated.

According to the invention, the two electrodes limiting the interaction space have adjacent surfaces of circular configuration, as viewed in cross section, with their convexities toward the axis of the tube, the object of such an arrangement being to ensure substantial equality or synchronism between the electron velocity and the phase velocity of the U. H. F. field throughout the entire interaction space.

The invention will be better understood from the following description, with reference to the attached drawings wherein,

Fig. 1 is a very schematic plan view representation of a circular structure tube of the conventional type.

Figures 2 and 3 show, in transverse and axial sections respectively, an embodiment of the tube according to the invention.

The tube, shown in Figure 1, comprises a cathode C located on a negative electrodes, which in this embodiment is inthe form of a cylinder, and a cylindrical anode coaxial with the negative electrode S and carrying delaying elements in the 'form ofa delay line. The magnetic field B is perpendicular to the plane of the figure, r being the anode, radius. M and N designate twolsuccessive positions of an electron emitted bythe cathode C- The distances between M and N and the tube axis are r, and r respectively.

- The electrode S and :the anode A may be considered as constituting a cylindrical condenser and the electric fields at M and N can be expressed as:

E0 being the intensity of "the electric field on the anode. Accordingly, electron velocity at M and 'N will be equal respectively to:

it a B B At a given point P along the path of the same electron located at a distance r from the axis, the electron velocity Vel is:

and

.t E e-e It may be easily seen that, at this point P, the phase velocity V,,, of the U. H. F. 'field of the wave which propagates along the anode A is V0 being the phase velocity of the wavealong the anode A.

Now, the delay line in such a tube is, according to the usual practice, so designed that the phase velocity .Vo of the wave is substantially equal to the ratio Now, since the value of r is always very close to that of r,,, it can be assumed:

r=r dr dr being very small. There-fore, it may be easily shown that:

Y g L'l ol :1 a

Since the last term of the right-hand side of the equation may be ignored, the equation may be rewritten K al Given a tube having an anode radius r =6 cm., and assuming that the electrons diverge from the anode by 0.2 cm., otherwise stated, that dr=0.2 cm., it may be Experience shows that the tube is unable to operate under such conditions, because a 5% discrepancy between the U. H. F. field phase velocity and the electron velocity makes a proper interaction between the U. H. F. wave and the beam impossible.

Figures 2 and 3 show an embodiment of the tube according to the invention in which this drawback is substantially eliminated.

Figure 2 shows an axial section of the tube comprising two circular electrodes 1 and 2 to which a potential dif- 1 ference isapplied by means of terminals 3. Electrode 1 is negative and electrode 2 is positive. The latter electrode is in the form of a delay line which, in the embodiment shown, is of vane type. Assuming the tube is operating as an amplifier, the input is coupled to a U. H. F.

source (not shown) by means of a loop 5. The output is coupled to a load by means of a loop 6.

In a groove provided in electrode 1, there is arranged a cathode 7 which is heated by a filament 8 and brought to the potential of the electrode 1 through a connection 9.

center M, the radius of the circle from center M for electrode 2 being designated by r.

Both circle segments are convex toward the tube axis.

Calculation and experience show that this arrangement results in that the ratio V V Vel is made substantially independent of r. In other words, with the arrangement according to the invention, phase velocity of the U. H. F. field provided by the wave travelling along the delay line 2 and the velocity of the electrons are substantially equal throughout the whole of the interaction space.

As a matter of fact, each portion of the interaction space between aparticular vane 4 and the electrode 1 may be considered as a portion of a cylindrical condenser ,the axis of which passes through the point M and is perpendicular to the plane of Figure 2. In this condenser, the anode is the inner armature, electrode 1 being the outside armature.

It has been shown (Figure 1) that, in conventional travelling wave tubes of circular structure, the anode constitutes the outside armature of a first similar cylindrical condenser. One will therefore realize that the curvature provided by the arrangement according to the invention at least partly compensates the effect of the curvature of electrodes 1 and 2.

Calculation would shOW that compensation is practically perfect if t0 the axis of the tube and r the radius of curvature of the arc of electrode 2 from the common center M in the plane of Figure 3.

Of course, the invention applies to any type of travelling wave amplifiers or oscillators of circular structure, with crossed electric and magnetic fields.

While it has been assumed in the above embodiment that the internal electrode was the negative one, it is obvious, that the invention equally applies if the internal electrode is the anode. The tube according to the invention is, in this latter case, established in the same manner as above, and the radius r of the meridian circle of the positive electrode will be equal to the half of the radius r What I claim is:

1. A travelling wave tube of circular structure which is energized by a source of potential comprising first and second electrodes in the form of bodies of revolution having a common axis, connections for connecting said first and second electrodes to the said source of potential for positively biasing said second electrode with respect to said first electrode to provide an electric field in the interaction space between said first and second electrodes, and means for providing in the space between said first and second electrodes a magnetic field substantially per pendicular to the electric field, the sections of said bodies of revolution disposed in planes containing said common axis being in the form of concentric arcs of circles, and the-arcs of the circles being convex towards the common axis to thereby provide substantial synchronisrn between the phase velocity of the U. H. F. field in one of said electrodes and the electron velocity essentially throughout the whole of said interaction space.

2. A travelling wave tube according to claim 1, where: in the radius of the arc of the circle of said second electrode is substantially half the minimum distance of the radius of said second electrode from said common axis.

References Cited in the file of this patent UNITED STATES PATENTS 2,600,509 Lerbs June 17, 1952 2,607,904 Lerbs Aug. 19, 1952 2,633,505 Lerbs Mar. 31, 1953 

